The present invention relates to a control system for a continuously variable belt-drive automatic transmission (CVT) for a four-wheel motor vehicle, and more particularly to a transmission control system for a four-wheel motor vehicle having a traction control system and an antilock brake system.
A known control system for a continuously variable belt-drive transmission comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to the servo device, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo device.
An electronic control system is provided for controlling the hydraulic circuit corresponding to the driving conditions of the motor vehicle. A desired transmission ratio is determined in accordance with vehicle speed and throttle valve opening degree to obtain the transmission ratio. Japanese Patent Application Laid-Open 64-52535 discloses such a system.
A four-wheel drive motor vehicle having a central differential is provided with a traction control system for restricting the differential operation in accordance with driving conditions, so that the output torque of a transmission is unequally distributed to front wheels and rear wheels of the vehicle.
Japanese Patent Applications Laid-Open 1-222364 and 1-222366 applicant disclose such systems.
The traction control system comprises a fluid operated multiple-disk friction clutch for restricting the differential operations, and an electronic control unit. The torque distributed to the front wheels and the rear wheels is controlled by controlling clutch torque of the friction clutch, thereby improving straight-ahead stability, accelerability and driveability.
If the front wheels slip or all the four wheels slip (slipping spin) at the same time on a slippery road, steering and driving of the vehicle become difficult.
In order to ensure a driving stability of the vehicle, the distribution of torque to the rear wheels is set to a larger value than that to the front wheels, so that the rear wheels may slip first. The traction control system (hereinafter called TCS) produces a control signal for controlling the torque distribution, thereby ensuring the driving force. Thus, the vehicle can be safely driven without a sudden change of driving stability.
In such a TCS, vehicle speed detecting means is provided for detecting vehicle speed based on speed of the output shaft of the transmission, in order to control the transmission ratio. However, if the front wheels or the rear wheels slip, the rotating speed of the slipping wheels, that is the speed of the output shaft of the transmission, does not correspond to the vehicle speed. Since the vehicle speed for controlling the transmission ratio does not correspond to the actual vehicle speed, the transmission ratio becomes unstable, and the line pressure reduces and causing slipping of the belt on the pulleys.
On the other hand, the antilock brake system (ABS) is provided for preventing the wheels from completely locking at rapid braking or at braking on slippery surfaces such as a snowy road, thereby ensuring directional stability and steering control of the vehicle during the braking operation and improving safety of the vehicle.
The ABS operates in such a manner as intermittently braking the wheels. Consequently, the rotating speed of the wheels, that is vehicle speed measured by the output shaft of the transmission changes at a short interval, and hence the transmission ratio fluctuates.